A land bridge is an isthmus or some other land-based connection between two otherwise disconnected islands or continents. Land bridges are important ecologically because they allow the exchange of plants and/or animals that are otherwise separated, sometimes by millions of years of independent evolution. Land bridges are often transient, appearing and disappearing over geological time due to rising and falling sea levels.
The most famous land bridge in Earth's history is probably the Bering land bridge, which existed from present-day eastern Russia to Alaska about 20,000 years ago, during the last glacial period. During this time, sea levels were approximately 120 meters (394 ft) below today's levels, due to the huge quantities of ice locked up in continental glaciers. The Bering land bridge allowed humans to migrate from Asia to the Americas. After they crossed over, it is thought that they migrated down the coastline.
Though the Bering land bridge is the most famous, there were several other land bridges in the world at the same time. These include the large tract of low-lying tundra in what is today the south North Sea, which has been dubbed Doggerland, after the Dogger Bank, a large sand bank in the area today. Doggerland connected together England with continental Europe, and a separate land bridge connected England to Ireland. The earliest inhabitants of these islands made it across via land bridges.
There were extensive land bridges between islands in present-day Indonesia, connected them to the southeast Asian mainland. These made it possible for early humans to travel from Africa to islands like Borneo. In what is the first confirmed instance of humans traveling over a significant stretch of open ocean, early man built rafts and made it across the present-day Wallace Line, a deep sea channel in central Indonesia that separates the fauna of west Indonesia (which is more Asian) from east Indonesia (more Australian). From the east side of the Wallace Line, these people reached New Guinea and Australia, which were also connected by land bridges.
Further back in the past, about 2 million years ago, one of the world's most ecologically significant land bridges formed -- that between North and South America. Unlike the other land bridges discussed, this was formed by continental drift rather than lowered sea levels. As South America had previously been isolated for tens of millions of years, this exposed the continent to the flora and fauna of essentially the rest of the world (Europe, Africa, Asia, and North America were all connected). Generally speaking, the North American fauna dominated the South American fauna, though some South American animals did thrive in North America, for instance, the opossum.